Ballast tube



J. KURSHAN Feb. 1, 1955 BALLAST TUBE Filed April 28, 1951 INVENTOR .kfw/1e Kursllall United States Patent O BALLAST TUBE `lerome Kurshan, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 28, 1951, Serial No. 223,559

2 Claims. (Cl. 201-63) The present invention relates to current-regulating devices, more particularly to devices of the type known as ballast tubes wherein an element of suitable resistance material (usually iron), suspended in an atmosphere of rariied gas (e. g. hydrogen or helium), serves to keep a current flowing through it at a substantially constant value over an extended range of voltages.

The regulatory effect of such tubes is due to the fact that a rise in applied voltage increases the dissipation of electric energy and, hence, the generation of heat in the element, the temperature coefficient of resistance of the element being positive and such as to cause its resistance to rise substantially in proportion to the voltage. Since, however, the change in the temperature of the resistance element is dependent not only upon the heat generated therein but also upon the heat carried off by radiation, convection and thermal conduction, the magnitude of the current drawn by the tube is not only a function of the applied voltage but also of the thermal coupling between the resistance element and the surrounding atmosphere.

Since the 'thermal coupling is, inter alia, determined by the pressure obtaining within the tube, it might be expected that variations in pressure would affect its current versus voltage characteristic; this is in fact borne out by experiments. There exists, however, no simple relation between pressure and thermal coupling; thus while the logarithmic temperature derivative of heat dissipation will generally increase with rising pressures, yet for very low pressures there exsts an anomalous region where the reverse is true. For a given pressure one can find, nevertheless, a predetermined voltage range within which ballast action, i. e. stabilization of the current, will occur.

The general object of the present invention is to provide a method of and means for extending the voltage range throughout which a substantially constant output current is obtainable, or, conversely, for reducing current variations within a given voltage range, as compared with conventional ballast tubes.

According to the invention I accomplish the above object by varying the thermal coupling between the resistance element of a tube and the tubes gaseous lling under the control of the voltage applied. Means for varying this coupling may comprise a current-responsive device connected in series or in shunt with the resistance element and adapted to vary, for example, the pressure inside the tube or the volume effectively occupied by the element, the latter expedient aiecting directly the rate of heat transfer between the element and the coolant atmosphere in which it is immersed. Where a pressure control means is provided, the same may operate either to raise or to lower the gas pressure with rising voltages, depending upon the pressure range for which the tube is designed.

The invention will be described in detail with reference to the accompanying drawing in which:

Figure l is a graph illustrating the behavior of a ballast tube at different temperatures and pressures;

Figure 2 is a graph illustrating the stabilizing effect of a ballast tube for two different pressures in a different pressure range from Figure l;

Figures 3, 4, 5 and 6 show, somewhat schematically, axial sections through different ballast tubes representing various embodiments of the invention.

Referring first to Figure 1, there is shown a diagram in which the abscissae represent the absolute temperature T in degrees K and the ordinates are either w=d (log W)/d(log T) or r=d (log R)/d(log T), R being the resistance of an iron wire in a ballast tube and W being the thermal energy dissipated into the surrounding atmosphere. 1t will be noted that there is only one curve for r whereas w assumes different values tor diierent pressures P. By way of illustration it may be mentioned that the two curves w'(P-=Pl arid w(P=P2) may be for pressures of 1 mm. Hg (Pl) and l0 mm. Hg (P2), respectively.

It can be shown that ballast action will occur as long as r w. For a pressure P1 the curves w and r intersect a first time at a relatively low temperature Tl and a second time at a higher temperature r13; t'or a pressure P2 the curves w and r intersect at a temperature T2 T1 and again at a temperature T 4 1 3. lt follows that for P=P1 ballast action will be obtainable at temperatures ranging from 'l'l to '13 whereas tor P=P2 the range will extend from T2 to T4. lf the pressure is changed from P1 to P2 as the voltage increases, the operating range can be extended trom a starting voltage corresponding to a temperature rl`l to a limiting voitage corresponding to a temperature T4, this range being evidently greater than that obtainable if the pressure is maintained at Pl, P2 or some intermediate constant value.

1n Figure 2, the abscissae represent the applied voltage V while the ordinates show the current l drawn by the tube. Between voltages V1 and V2 the current rises gradually along curve l if the pressure is maintained at a value P=P3, and along a similar curve l, corresponding to higher values, it the pressure is at P=l4; it is assumed that both values P3 and P4 are substantially higher than the value P2 of Figure l, thus greater than l0 mm. Hg, and that P4 P3. lt will be seen that the current may be maintained at a substantially constant value IR it` the pressure is gradually changed from P4 to P3 as the voltage rises from V1 to V2.

Ari arrangement embodying the invention is shown in Figure 3; the ballast tube 30 includes input terminals 31, 32, an envelope 33 enclosing a raried gas G such as, for example, hydrogen or helium, and a resistance element in the form ot a coiled wire 34, e. g., of iron, surrounded by the raried gas and connected to terminal 31. The tube also includes means for varying the thermal coupling between the wire coil 34 and the gaseous atmosphere, in the form of a bimetallic strip 35 having one end rigidly supported by a conductive rod 36 which in turn is connected to terminal 32, the coil 34 being anchored to the other, free end of strip 35.

When a voltage, e. g., as indicated in the drawing, is applied to the terminals 31, 32, current will ow in a series circuit consisting of rod 36, strip 35 and element 34, the latter being initially contracted as shown in full lines. In this condition of the coil there is less thermal coupling between it and the surrounding atmosphere than for more extended conditions, therefore the wire 34 heats up relatively rapidly to reach at low voltage the point at which ballast action starts. From this point on, the current will increase at a slow rate which, however, will be sufcient to cause a gradual deformation of the bimetallic strip 35 into the position shown dot-dash lines, this resulting in a corresponding expansion of the coil 34. Owing to this expansion the coupling between coil 34 and the atmosphere increases, more heat is dissipated and the temperature of the wire rises slowly toward a point at which the current departs from its substantially constant value. It will thus be seen that this arrangement alfords current stabilization over an increased range of input voltages.

While the element 34 has been described above as an iron wire, greater elasticity of the coil may be obtained through the use of a more resilient material as a core supporting the iron in the form of a coating thereon. A suitable material for this purpose is tungsten; since this, however, is a conductor, an insulating layer (e. g. of aluminum oxide) must be interposed between the core and the iron. It may also be mentioned that where it is possible to use the tube 30 exclusively in the vertical position illustrated, the resiliency of the wire 34 will be of less importance inasmuch as the turns of the coil 34 will tend to contract under their own Weight; they may, of course, be additionally loaded to increase this tendency.

In the modification of Fig. 4 there is shown a tube 40 comprising terminals 41, 42, envelope 43 and resistance element 44, the latter being in the form of a simple wire loop having one leg connected to terminal 42. Connected in series between the other leg of wire 44 and terminal 41 is a solenoid coil 45 controlling a plunger 46 which in turn operates a bellows 47 enclosing the loop 44. The interior 48 of the bellows 47 contains a rareed gas G while the rest of the tube may include a similar or diterent gas G preferably under approximately the same pressure, a slight but variable pressure differential between the chamber 48 and its surroundings being brought about by expansion or contraction of the bellows under the control of solenoid 45, 46.

Depending on whether the device operates in a pressure range as indicated in Figure 1 or as indicated in Figure 2, the solenoid may be arranged either to compact or to expand the bellows 47 as the current through the wire 44 and, with it, through the coil 45 increases. It is, of course, again understood that a relatively small increment of current should suffice to bring about the desired control action, the initial pressure inside chamber 48 being for this purpose so selected as to vary critically from, say, P4 to P3 (Fig. 2) as the input voltage rises from V1 to V2.

In Figure use is made of the thermal characteristics of a reversible adsorbent for gas, such as zirconium, to achieve the desired change in pressure. This material has the property of absorbing hydrogen gas at relatively low temperatures and to release it when heated to higher temperatures. In Figure 5 the tube 50 comprises terminals 51, 52, an envelope 53, a resistance wire 54 extending between the terminals, and a control member 55 of zirconium exposed to thermal radiation from wire 54; the member 55 may be in the form of a tube spacedly surrounding the wire. When the wire 54 is traversed by current, and the initial pressure has a value of, say, P1 (Fig. 1), then ballast action will start when the wire reaches a temperature T1. Somewhere between this temperature and a temperature T3, at which ballast action would normally end as shown in Figure 1, the zirconium member 55 is heated sufficiently to release some gas which will increase the pressure to a value P2, thereby extending the range of current stabilization to a point corresponding to a temperature T4. that even with an almost constant current I the rise in voltage V may sufce to create enough additional heat to bring about this change in gas pressure.

In the arrangement of Figure 6 the ballast tube is associated with an ionic pump of the type disclosed in U. S. Patent No. 2,282,401, issued May 12, 1942. Across the input terminals of the tube 60, indicated diagrammatically at 61 and 62, is connected the resistance element 64 and in shunt therewith a magnet coil 65 surrounding the central portion of the tube envelope 63 which is internally lined with an anodic coating 66. This central portion is shielded from the right-hand end of the tube, containing the resistance wire 64, by a metal screen 67 and from the left-hand end of the tube, enclosing a cathode 68, by an apertured plate 69. The input voltage is applied to the terminals 61, 62 from a source 70 in series with a load 71, the magnet coil 65 being, if desired, in series with a resistor 72 designed to It should be noted limit and/or adjust the current drawn by this coil. Operating voltages for the cathode 68 and the anode 66 are derived from batteries 73 and 75 respectively.

In operation, electrons emitted by the cathode 68 are drawn toward the anode coating 66 and ionize the gas within the envelope 63, the paths of the electrons being indicated at 74. Ions produced by collision of gas molecules with the electrons are attracted by the cathode 68 and pass through the aperture of plate 69, thereby lowering the lgas pressure inside the part of the tube to the right of that plate. Thus the pressure around the wire 64 will assume a value such that the pumping action due to the leftward electron injection movement of positive ions is just balanced by diffusion of neutral atoms in the opposite direction. Since, however, the magnetic tield produced by the coil 64 will tend to lengthen the electron paths (as more fully set out in the patent referred to), the number of collisions will be increased and the pressure level around wire 64 will be reduced as the voltage across terminals 61, 62 rises, hence stabilizing action as described in connection with Figure 2 will again occur. Since the control current (through magnet coil is here independent of the current through the resistance element, the latter may, in fact, be made to decrease with increasing voltages to compensate for the rise in the current drawn by the circuits 65, 72 whereby the total current through the load 71 may be held constant. It will, of course, be understood that a similar shunt arrangement may be substituted4 for the series arrangements shown in Figures 3 an The various embodiments described above and shown in the drawing have been given merely for purposes of illustration and not as a limitation upon the scope of the invention which, on the contrary, is capable of numerous adaptations and modifications within the scope of the appended claims.

What is claimed is:

l. A ballast tube comprising an envelope, a gas in said envelope, input terminals, a resistance element in said gas connected between said input terminals, pressure regulating means adapted to vary the pressure of said gas, and electric control means for said pressure regulating means, said control means being connected f between said input terminals and in series with said resistance element for actuation by a voltage applied there- 2. A ballast tube comprising an envelope, a gas in said envelope, input terminals, an expandible resistance element in said gas connected between said input terminals, and a bimetallic strip mechanically secured to said element for varying the expansion of said resistance element, said bimetallic strip being connected between said input terminals and electrically in series with `said element for actuation by a voltage applied thereto.

References Cited in the tile of this patent UNITED STATES PATENTS 

